You wrote in article <n0alvp4j.fsf@ccs.neu.edu> in gmane.comp.lang.lightweight: > The reason there are no `TRULY heterogeneous lists' is because it is > always possible to create a discriminated union type that covers all > elements. > > It is especially pointless if you fold all operations on objects > into the object interface itself. > > But if you take the view that certain operations (like intensional > equivalence or intensional identity) are independent of the objects, > then the notion of heterogeneous lists becomes much more interesting. > Permutation, for example, becomes possible. You seem to be claiming that permutation of a truly heterogeneous list is impossible without taking the view that certain operations (like intensional equivalence or intensional identity) are independent of the objects. I probably misunderstood you, but I don't see how this claim is true, if you are in fact making it. Permutation of lists can be done for a homogeneous list without knowing what type all of the objects in the list are (the type of such a permutation function looks like "forall a. [a]->[a]"); as a special case, one can permute a list whose element type is homogeneously "I have no idea" (substitute "exists b. b" for "a"). -- Edit this signature at http://www.digitas.harvard.edu/cgi-bin/ken/sig Sexy types in action: http://www.eecs.harvard.edu/~ccshan/cs252/usage.pdf new journal Physical Biology: http://sansom.biop.ox.ac.uk/images/physbio.gif What if All Chemists Went on Strike? (science fiction): http://www.iupac.org/publications/ci/2003/2506/iw3_letters.html
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